honest to God

Etymology
Compare. Originally used as a sentence adverb or interjection, modifying entire clauses or utterances; itself is used this way, as in “Honest, I had no idea!” (meaning “I'm telling the truth, I had no idea!”).

Adverb

 * 1) Honest; truly, really;
 * Honest to God, I thought you already knew!
 * 1) Truly, really.
 * She is honest to God brilliant.